This invention relates to a synchronous digital communications system for optically transmitting electric signals and to a method of optically transmitting electric signals.
A synchronous digital communications system is based, for example, on a standard for synchronous digital hierarchy (SDH/SONET standard). In such a digital communications system, individual network elements are interconnected by different transmission media (e.g., copper cables, optical fiber waveguides, or radio links). A network element is, for example, an exchange for a public switched telephone network, a cross-connect, or an add/drop multiplexer. To synchronize the network elements, two techniques are known: master-slave synchronization and mutual synchronization.
The master-slave technique, also referred to as hierarchical synchronization, uses a unique primary reference clock for synchronization of a first hierarchical level of network elements, also referred to as nodes. These nodes give their derived clocks to the next level nodes, and so on. In the mutual synchronization technique, all nodes are at a peer level interconnected by the existing digital links. Each node calculates a mean phase value of the incoming clocks and its own internal clock.
From DE 44 46 511 it is known to avoid timing loops by grouping interface units of each network element that are used for synchronization in two classes, thereby defining a synchronization hierarchy. The interface units of one of the classes ignore received synchronization signals, and the interface units of the other class transmit synchronization signals (clock references).
Network elements have a number of interface units, which generally all serve to receive and transmit information signals, i.e., speech, data. Some predefined interface units additionally serve to receive and/or transmit synchronization signals. All-electric synchronous digital communications systems have nonswitched physical connections. A synchronization hierarchy is defined by predetermined paths. If section-by-section radio or point-to-point optical transmission is used, the electric signals (information+synchronization) are switched through transparently. In this way, the network element interface units used for synchronization always receive the necessary synchronization signals. Even if no information is transmitted in the meantime, the connections between the network elements are maintained, for example by transmitting default messages, so that continuous synchronization is ensured.
A new situation arises if during section-by-section optical transmission, no time-invariable through-switching takes place. Then, optical connections are no longer permanently assigned to wavelengths. A flexible and time-variable assignment of optical channels to wavelengths is possible. For example, an optical channel for transmitting a first message packet is implemented by a first switched optical connection using a first wavelength, and an optical channel for transmitting a second message packet is implemented by a second switched optical connection using a second wavelength. If network elements with switching properties, such as optical cross-connects, are used in conjunction with wavelength-division multiplexing, arbitrary, time-variable optical channels can be created for transmitting information signals, such as SDH or SONET signals. For example, a first optical connection for creating a first optical channel is used in a first time period to transmit messages from a first network element to a second network element, with an optical cross-connect interposed between the network elements. The first optical connection is implemented using a first wavelength, for example. Via the interface unit assigned to the first wavelength, the second network element synchronizes itself, i.e., the synchronization clock, which corresponds to a bit-rate clock, is used for all interface units of the second network element. If in a second time period, the optical cross-connect uses the first wavelength for a second optical connection to create a second optical channel for transferring information from the first network element to a third network element, the connection to the second network element via the first wavelength is interrupted. The second network element can no longer synchronize itself in the second time period. Even if the second network element received information and/or synchronization signals over a second or third optical connection, it could not synchronize itself, because only the interface unit assigned to the first wavelength is reserved for the purpose of accomplishing synchronization for all interface units. Instead of using one interface unit, synchronization can also be achieved using two or three interface units, for example by means of an additional selection facility that selects the clock of the best quality. Through the use of three interface units for synchronization purposes in conjunction with three wavelengths, the probability that no synchronization is possible can be minimized but cannot be reduced to zero.
The invention proposes a synchronous digital communications system and a method of optically transmitting electric signals, as described more fully below.
The synchronous digital communications system serves to transmit electric signals optically. The electric signals to be transmitted are converted from electrical to optical form and are then transmitted using wavelength-division multiplexing (WDM) or dense wavelength-division multiplexing (DWDM). Using at least one wavelength per transmission section between optical network elements or between optical and electrical network elements, at least one optical connection is implemented as a nonswitched connection and serves to transfer synchronization and information signals. This has the advantage that independently of the switched communication links, synchronization is constantly ensured throughout the system. Each network element has at least one interface unit that is reserved for synchronization and that constantly receives signals on the wavelength reserved for synchronization.
The synchronous digital communications system comprises, for example, at least three network elements interconnected by optical lines, each of the network elements comprising at least one electrical-to-optical converter and at least one optical-to-electrical converter. At least one optical cross-connect is connected between the network elements. Each optical cross-connect is adapted to use optical connections with the exception of the at least one nonswitched optical connection to route signals from one network element to another. The cross-connect performs switching operations for communication links. With respect to the switching, however, the cross-connect is limited to the existing wavelengths less the wavelengths used for synchronization.
In a preferred embodiment, the synchronous digital communications system comprises at least three network elements designed as SDH or SONET elements that are interconnected by optical lines. Between the network elements, hierarchical synchronization is established by configuring at least one optical connection as a nonswitched connection for transmitting a synchronization clock generated in a primary reference source, and clocks derived therefrom, over predetermined paths. For instance, a reference clock generated in a first network element is transmitted for synchronization purposes to a second network element using a first reserved, nonswitched wavelength. A clock derived from the received reference clock in the second network element is transmitted to a third network element using a second reserved, likewise nonswitched wavelength. Between the first and second network elements, the first wavelength is then reserved for the transmission of synchronization signals but can simultaneously be used for the transfer of information, maintenance and/or management signals. All other available wavelengths, e.g., twenty wavelengths, are used for the transfer of information signals over switched optical connections. Between the second and third network elements, the second wavelength is then reserved for the transmission of synchronization signals and can additionally be used for the transmission of information. Regardless of whether information is being transmitted at the second wavelength, synchronization signals are always transmitted. All other available wavelengths, e.g., the first and the third through the twentieth wavelengths, are used for the transfer of information signals over switched optical connections. Synchronization is guaranteed throughout the system.
Alternatively to hierarchical synchronization, the invention can also be used with mutual synchronization. The synchronous digital communications system comprises at least three network elements designed as SDH or SONET elements that are interconnected by optical lines. Between the network elements, mutual synchronization is implemented by configuring at least one optical connection as a nonswitched connection for transmitting at least one synchronization clock generated in at least one primary reference source over predetermined paths. In determining the paths, care must be taken to ensure that no timing loops are created. In each of the paths, at least one selected wavelength is used exclusively for the transfer of synchronization signals. Thus, there is at least one nonswitched auxiliary optical connection on each link between two network elements that is used for synchronization purposes and can additionally be used to transfer information, maintenance, and/or management signals.
In another preferred embodiment of the invention, the synchronous digital communications system comprises at least three network elements as well as a synchronization manager and a connection manager, the network elements being interconnected by optical lines. The synchronization manager is adapted to configure dedicated, nonswitched synchronization links between the at least three network elements. The connection manager is adapted to configure communication links from a pool of optical connections in which the dedicated synchronization links are marked, without switching the synchronization links. The synchronization manager and the connection manager perform network management functions. During system design, the number of network elements, the number of possible optical connections, etc. are determined. For the synchronization, a topology is defined in the synchronization manager. For instance, master-slave synchronization is chosen. To implement this synchronization, the necessary paths are determined. For each path, a nonswitched optical connection is configured using at least one wavelength. In each network element, at least one interface unit is selected for synchronization. Each of the selected interface units is assigned a nonswitched wavelength for synchronization purposes. This wavelength and the associated path are configured to create a nonswitched path over which synchronization signals are constantly transmitted. After completion of the configuration of the synchronization links, the configuring of the communication links takes place. Information is transmitted over switched communication links and can additionally be transmitted over the synchronization links. The wavelengths reserved for the synchronization links are usable by the connection manager only limitedly, because they are not switchable.
In the novel method of optically transmitting electric signals, the electric signals to be transmitted are converted from electrical to optical form and then transmitted using wavelength-division multiplexing, with at least one optical connection being configured as a nonswitched connection using at least one wavelength and serving to continuously transmit synchronization signals, and at least one further optical connection being switched using at least one further wavelength. Each switched optical connection is used for the transfer of information signals. The method can be used in a synchronous communications system with at least three network elements interconnected by optical lines. Dedicated, nonswitched synchronization links are then connected between the at least three network elements for the permanent transmission of at least one synchronization clock.